Lithography is a printing system taking advantage of the natural mutual repulsion between water and oils. The surface of a lithographic printing plate comprises areas which accept water and repel an oily ink, and areas which repel water and accept an oily ink. The former areas constitute non-image areas, and the latter areas constitute image areas. Accordingly, if the surface properties between these two areas is disturbed, for example, if the hydrophilic property of the non-image areas is deteriorated for some reason, inks are apt to adhere to such hydrophilicity-deteriorated areas to cause background stains.
Such background stains are formed under various conditions. Typically, background stains are formed in the case where a lithographic printing plate is subjected to a burning-in treatment for the purpose of imparting high printing durability (i.e., run-length of printing) or in the case where the surface of a lithographic printing plate is allowed to stand in air without protection with a desensitizing gum.
These phenomena sometimes occur even when operation of a printing machine is suspended, e.g., because of disorders during printing or for recesses. Therefore, it is common to apply a desensitizing gum onto the printing plate during suspensions of printing. Further, when a lithographic printing plate unprotected with a desensitizing gum is left to stand with oleophilic substances being attached to the non-image areas thereof, such contaminated areas are rendered oil-sensitive, thus resulting in formation of stains. For example, the appearance of fingerprints in the background of prints is ascribed to this phenomenon. Furthermore, when the non-image areas take scratches, the scratches are filled with an ink and are gradually rendered oil-sensitive to cause stains.
In these cases, it is usual that a printing ink is first removed from the lithographic printing plate and the stained plate is then treated with a plate cleaner for restoring the hydrophilic property of non-image areas.
Such a plate cleaner conventionally includes a cleaner comprising an aqueous solution of sodium silicate. However, this plate cleaner, despite of its very high desensitizing activity, is disadvantageous in that a part of an image is damaged or adhesion of an ink to image areas is deteriorated because of its alkaline property when it is applied to lithographic printing plates produced from presensitized lithographic printing plate precursors such as positively-working presensitized lithographic printing plate precursors having a light-sensitive layer composed of an o-quinonediazide compounds, as described, e.g., in Japanese Patent Publication No. 28403/68, U.S. Pat. No. 3,046,120, etc., negative-working presensitized lithographic printing plate precursors having a light-sensitive layer composed of a binder having an acid group and a diazo resin, and the like, that are developed with an aqueous alkaline developer.
Furthermore, a plate cleaner using oxalic acid, as disclosed in U.S. Pat. No. 3,489,561, has a weak desensitizing property and an activity to corrode a metal support. Therefore, when it is applied to commonly employed lithographic printing plates comprising an aluminum plate as a support, the hydrophilic layer provided on the support, for example, a layer formed by a surface treatment to render hydrophilic, as described in U.S. Pat. No. 2,714,066, is susceptible to destruction to cause stains. Therefore, such a plate cleaner is not adequate for metal supports.
In general, when stains generate during printing, the printing plate surface is first treated with an ink remover (e.g., kerosene or hydrocarbon type solvents) to remove the ink, and then with a desensitizing agent. The above-described plate cleaner is also used as a desensitizing agent after the removal of ink. This means that the treatment for cleaning the plate surface should be done over two steps. Therefore, an emulsion type plate cleaner having both functions, i.e., serving as an ink remover and also as a desensitizing agent, has recently been developed. For example, Japanese Patent Application (OPI) No. 15702/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses an emulsion type alkaline plate cleaner, and Japanese Patent Application (OPI) No. 2102/78 discloses an emulsion type acidic plate cleaner.
However, these emulsion type plate cleaners have disadvantages of troublesome handling, inferior workability, and high cost.